This laboratory continues to devote its primary effort to the study of immunoglobulin structure and function and development of methodologies for protein sequence analysis of cells involved in the immune system. Specifically, during the next year the following projects will be continued or undertaken: 1. To define the necessary sequence for complement fixation in the C micron 4 domain of IgM. 2. To determine necessary configurational constraints for Cl fixation in the C micron 4 domain. 3. To complete the primary structure of J-chain. 4. To determine sulfide intrachain linkages within J-chain and to define those cysteines which bind to the penultimate cysteine of micron chain (i.e., two separate cysteines within J-chain should be involved in this binding).